In many radio communication systems, signals are propagated along several paths between its source and its destination. If the source and destination are stationary, optimum results may be obtained by proper antenna location. Mobile systems, however, are subject to uncontrolled multipath propagation. The signal at the destination is usually a composite of two or more differing versions of the signal applied to the source antenna. For high frequency channels, some or all versions of the transmitted signal are reflections from fixed or moving obstacles which arrive at the destination at different times. The radio channel is frequently subjected to severe multipath fading so that the signal to noise ratio deteriorates. As is well known in the art, mobile systems are subject to short and deep fades generally termed Rayleigh type fading. While the quality of voice communication is affected by the multipath fading in mobile systems, the received signal is usually intelligible. For digital signals, fading increases the error rate. The larger error rate in turn increases number of data retransmissions, increases the number of handoffs for voice or raises the power level needed for adequate reception.
As is well known in the art, fading can be mitigated by spatial diversity. U.S. Pat. No. 3,864,633 issued Feb. 4, 1975 to H. Strenglein discloses a space diversity communication system in which signal fading is reduced through use of closely spaced antenna sensors. Sum and difference signals formed from the outputs of the antenna sensors are analyzed to select or combine the antenna outputs or to electronically steer the system receptivity pattern.
U.S. Pat. No. 4,271,525 issued Jun. 2, 1981 to K. Watanabe discloses an adaptive diversity receiver for digital communications in which a plurality of transversal filters are disposed in front of a linear combiner. The tap gains of each transversal filter are updated to estimate the sampled values of the impulse response for the channel as a function of the detected data output rather than as a function of the data error signal. The transversal filters operate as adaptive matching filters for the diversity channels.
U.S. Pat. No. 4,752,941 issued Jun. 21, 1988 to S. Takahara et al discloses a diversity receiving system in which a phase shift circuit adjusts the phase of one or more of a plurality of received signals to improve signal combining. U.S. Pat. No. 4,731,801 issued Mar. 15, 1988 to J. Henriksson discloses a method for reception and detection of digital systems using diversity. The demodulated in-phase baseband signals and the quadrature baseband signals are separately summed and detected in-phase and quadrature phased signals are obtained therefrom. This optimizes recpetion by adjusting the phases of the received signals using mean square deviation or signal projection criteria.
The foregoing techniques require relatively complex arrangements to adjust the phases of the signals from the plurality of antennas. U.S. Pat. No. 4,675,863 issued to E. Paneth et al Jun. 23, 1987 and U.S. Pat. No. 4,817,089 issued to E. Paneth et al Mar. 28, 1989 disclose a spatial diversity system wherein a spatial diversity combiner circuit receives demodulated data symbols, AGC values, signal+noise, magnitude and phase error and determines the best demodulated symbol using a majority vote and calculations of signal to noise ratios for each modem. The complexity of signal co-phasing is replaced by majority voting of separately derived symbols based on a complex variety of factors rather than on a direct analysis of the diversity signals. Consequently, the characteristics of the differential phase component signals are not used for multipath fade distortion correction.
U.S. Pat. No. 4,397,036 issued Aug. 2, 1983 to K. Hirade et al discloses a diversity system utilizing a plurality of branches for an angle modulated digital signal transmission which avoids phase adjustments of received signals. Rather than co-phasing, the received signal on each branch is modulated with orthogonally related local signals having the same period as the digital signal. The modulated signals are combined by simply summing them, and the combined signal is differentially detected. While co-phasing of received signals is not required in U.S. Pat. No. 4,397,036, the restrictions on the frequency of the local signal limit the application of the technique.
In many mobile systems such as cellular telephone, it may be necessary to retune the receiver system to different channels at nearby frequencies. In such systems, it is difficult and more expensive to maintain the needed relationship between the received and local signals of Hirade. It is an object of the invention to provide an improved diversity arrangement for digital signals which avoids the complexity of co-phasing, prescribed relationships between the received and local signals and complex processing of demodulated symbols.